Those of us who donate whole blood and platelets on a regular basis can do so in large part because of hematopoietic stem cells (HSC) in our bone marrow, which are capable of giving rise to all the blood cell lineages (e.g. erythrocytes, lymphocytes, thrombocytes) and can maintain their own numbers through self-renewal. Naturally, the production and balance of appropriate numbers of different blood cell types in healthy individuals, in response to infection, and following blood donation require regulated expression of genes critical for differentiation and for stem cell properties. The Polycomb Group of genes, originally identified in Drosophila, encode proteins that modify histones, thus altering chromatin structure and, ultimately, gene transcription. Homeotic genes, which influence anterior-posterior patterning in Drosophila as well as vertebrate embryos, are suppressed by Polycomb Group (PcG) proteins, and mutations in PcG genes result in changes in the identities of body segments (Lessard and Sauvageau, 2003).
More recently, the roles of PcG proteins in maintaining self-renewing mammalian stem cells have become an important research focus. PcG proteins are present at high levels in the totipotent early blastomeres and inner cell mass of mouse embryos, and decline in amount as the descendants of these cells begin to differentiate. Maintenance of adult stem cells, particularly in the hematopoietic compartment, is also likely to require silencing of differentiation genes by PcG proteins (Rajasekhar and Begemann, 2007). Null mutation of PcG genes in Polycomb repressive complex 1 (PRC1), such as Bmi-1, causes defects in hematopoeitic stem cell proliferation, leading to aplastic anemia and susceptibility to infections. Conversely, Bmi-1 can promote tumor development by cooperating with another oncogene, c-myc, to generate B- and T-cell lymphomas (Lessard and Sauvageau, 2003).
The consequences of too much or too little PRC1 function for blood cell development and leukemia clearly reflected the roles of this protein complex in maintaining the gene repression required to keep hematopoietic stem cells (HSC) in a self-renewing proliferative state. The second complex, PRC2, includes Embryonic ectoderm development (Eed), Enhancer of zeste 2 (Ezh2), and Suppressor of zeste 12 (Suz12) proteins. Mice that have reduced amounts of Eed protein (hypomorphic allele) have proliferation defects in myeloid and lymphoid lineages that progress to leukemia (Lessard and Sauvageau, 2003). This month in PLoS Biology, Majewski and colleagues reported on the consequences of reducing amounts of PRC2 on another hematopoietic lineage, platelet formation, and their studies were initiated by isolating a mutation in the PRC2 gene Suz12.
The Plt8 mutation rescues hematopoietic colony-forming ability in thrombocytopenic mice (Majewski et al., 2008 ). A. Numbers of colony-forming units in recipient spleens B. Photos of recipient mouse spleens
This paper is noteworthy for the genetic screens, functional assays, and genomic techniques employed to examine the function of PRC2, as well as for the utilization of a mammalian experimental model, and the potential for applying the findings to development of cancer treatments. The investigators were interested in finding mutations that suppress the stem cell abnormalities and/or reduced platelet counts (thrombocytopenia) in mice that have defects in thrombopoetin signaling, which is critical for normal platelet production. Through an extensive and elegant series of genetic experiments, Majewski and colleagues identified a mutation in the Suz12 gene (Plt8) that “rescued” platelet counts in c-Mpl-/- mice. The numbers of HSC were then determined, using the classic method of donor bone marrow transplantation into lethally-irradiated recipient mice. The Plt8 mutant bone marrow contributed more hematopoietic progenitors than did wild-type bone marrow in both competitive and serial transplantation experiments.
These results with the Plt8, however, reflected only a partial loss of Suz12, and so to reduce expression of this gene further, Majewski and colleagues used short hairpin RNA-mediated silencing. In this case, a nonspecific sequence shRNA could be used as a control, and again, the Suz12-deficient bone marrow contributed more colony-forming blasts and megakaryocyte progenitors in transplantation experiments. The points made in the discussion section of this paper emphasize the importance of the results to understanding PRC2 function, in the context of hematopoiesis. First, altering levels of one PRC2 component (Suz12) changed the levels of the other components (Eed and Ezh2), and reduced total amounts of the complex in hematopoietic cells (thymocytes and splenocytes). Second, the reduction in PRC2 resulted in changes in the expression of three cancer-related genes: Bex2, Bex4, and Fibulin. Third, manipulating the amount of PRC2 may have therapeutic applications for bone marrow transplantation and cancer treatment. Overall, this paper is a tour de force of mouse genetics, functional hematopoietic assays, and gene expression analyses, and my only quibble is that potentially conflicting results from Kamminga and colleagues (2006), on the overexpression of Ezh2 and inhibition of replicative senescence in HSC, are inadequately discussed.
Kamminga, L.M., Bystrykh, L.V., deBoer, A., et al. (2006). The Polycomb group gene Ezh2 prevents hematopoeitic stem cell exhaustion. Blood 107, 2170-2179.
Lessard, J., and Sauvageau, G. (2003). Polycomb group genes as epigenetic regulators of normal and leukemic hemopoeisis. Experimental Hematology 31, 567-585.
Rajasekhar, V.K., and Begemann, M. (2007). Concise review: roles of Polycomb Group proteins in development and disease: a stem cell perspective. Stem Cells 25, 2498-2510.
Majewski, I.J., Blewitt, M.E., de Graaf, C.A., McManus, E.J., Bahlo, M., Hilton, A.A., Hyland, C.D., Smyth, G.K., Corbin, J.E., Metcalf, D., Alexander, W.S., Hilton, D.J., Goodell, M.A. (2008). Polycomb Repressive Complex 2 (PRC2) Restricts Hematopoietic Stem Cell Activity. PLoS Biology, 6(4), e93. DOI: 10.1371/journal.pbio.0060093